feat: keep proxy forwarding responsive under capture pressure

[kev1n77]

Summary

• add a bounded IPC failure circuit that drops capture backlog without blocking proxy forwarding and automatically probes recovery after 10 seconds
• extend the packaged sidecar probe with a synchronized client cancellation and 36 concurrent requests producing 72 events while IPC accepts but never acknowledges
• require cancellation/backpressure evidence in Python bundle validation and the Rust desktop runtime contract
• update TST-002 progress and runtime documentation

Verification

• python -m unittest discover -s sidecars/mitmproxy/tests -v (21 passed)
• npm run sidecar:package
• npm run sidecar:verify
• packaged integration probe: cancellation survived, 36 responses remained correct under IPC backpressure, bundle ready
• cargo fmt --all -- --check
• git diff --check

Full local Rust workspace/Tauri execution remains blocked before compilation by the host's incomplete Perl runtime required by vendored OpenSSL; the PR matrix provides the authoritative full cross-platform run.